1. Field of the Invention
This invention relates generally to dry rotary displacement compressors and blowers and in particular to screw compressors using oil for lubrication, cooling and sealing to produce oil-free compressed gases, and wherein the compression space produces oil-free compressed gases.
2. Background Art
Rotary displacement compressors include sets of rotary gear wheels that must be maintained lubricated and cool in order to provide the compressive power while maintaining their integrity. In some prior art devices, for example those described in U.S. Pat. No. 3,976,165 to Pilarczyk, high pressure oil was used to lubricate carbon seal rings in a contacting labyrinth gas seal. Beyond the complexity and expense of such a device, the oil seal was a “wet seal” in which the gas being compressed, referred to as the process gas, came into contact with the lubricating oil and some became entrained in the process gas resulting in an undesirable condition.
Oil enters the compression space through gaps that form between the shafts connected to the rotors and the wall of the casing or housing enclosing the shafts. In a space where the gas is being compressed by rotors, it is important that no oil is present. For dry type (oil-free type) seals that provide a seal between the compressor elements and the remainder of the pump, the elements are provided in a housing, separated from the compressor elements. Nevertheless, the risk of oil used for lubricating the journaling rotor bearings entering the compression space remains, and after entering the compressor portion, the high rotational speed of the rotors causes any lubricating oil to mist up and to become entrained in the gas being compressed. This is highly undesirable when clean “dry” compressed gas is needed.
The lubricating oil, although critical in maintaining the shaft journaling rotor bearings lubricated, must be sealed in the bearing chamber or otherwise maintained away from the compression space where the impeller rotors are rotating to compress the process gas. Thus, it has hitherto been necessary to provide a seal that prevents the oil from migrating from the bearings toward the compression elements in the housing and toward the environment.
The usual practice to accomplish this sealing function is to provide an oil shield between the shafts and the compression space. Such oil shields are described, for example, in U.S. Pat. No. 4,487,563 to Mori et al., and in several Japanese patent published applications cited within that U.S. Pat. No. 4,487,563 discloses an oil-free rotary displacement compressor including a main casing defining a compression space, a pair of rotors housed in the compression space and each having a screw thread portion and shaft portions, radial bearings and thrust bearings for causing the rotors outer diameter vanes to mesh, shaft sealing means for sealing the shaft portions to avoid gas leaks from the compression space to outside and entry of oil into the compression space from the bearings. Floating type screw seal rings are each formed on the inner surface with a spiral screw thread and a root of thread. These are fitted over one of the suction-side shaft portions and supported by the casing in such a manner as to be movable in both the radial and the axial directions with respect to the casing. Stationary type screw seal rings, each formed with a spiral screw thread and a root of thread are each fitted over one of the discharge-side shaft portions and supported by the casing in such a manner as to be movable only in the axial direction with respect to the casing. For purposes of description, the teaching of the shaft and rotor configuration found in U.S. Pat. No. 4,487,563 is incorporated herein by reference.
Other prior art attempts directed toward providing for oil shield integrity and effectiveness have not been completely successful in that the design of the oil shield and sealing methods are cumbersome and complicated and do not always effectively repel oil form reaching the compression space or chamber. For example, the sealing capability of the rotary screw seals used in most dry rotary screw compressors and blowers rely primarily on the positive displacement pressure of the air in the compression chamber to drive back the oil that lubricates the bearings just outside of the rotary screw seal sets. However, because of the need to have the pressure at a high level in order to provide a sufficient barrier to oil leakage, too much of the compressed gas is evacuated from the compression chamber and permitted to leak outwardly back toward the bearings. As the bearings are usually in fluid communication with the environment, the operational efficiency of the compressor is decreased, and a problem with an oil seal in the bearings can also cause oil to leak out of the bearing chamber under the high pressure process gas.
Additionally, the location of the oil shield and the construction of the oil shield means are not particularly suitable for providing ideal conditions for providing sufficient sealing capacity of the screw type seals to repel oil leakage past the seal. The invention applies primarily to positive displacement dry rotary screw compressors and blowers where oil is not introduced to the compression chamber for sealing, cooling, or lubrication. The efficiency and performance of such equipment is greatly dependent on effective sealing of the pumping chamber against process gas leakage and the ingress of lubricating oils used on the gears and bearings.
Sealing methods used to accomplish the above traditionally include lip seals, mechanical face seals, and non contact labyrinth seals. It is often desirable to increase the rotation speed of these compressors and blowers to achieve increased pumping efficiency. These performance requirements often exceed the maximum speed capabilities of lip seals, mechanical seals, and any type of seal which requires contact between stationary and rotating elements to establish sealing. This leaves only non-contacting labyrinth seals as an alternative. Most traditional labyrinth seals have performance limitations in speed, pressure sealing capability, and service temperature which are addressed in the present invention.
U.S. Pat. No. 7,338,255, among others, proposes a method of restraining oil flowing into the compression chamber by providing a series of stepped ledges and very narrow rotation clearance between the rotor and the stator of an impeller to provide for sealing. This seal relies on the principles of a labyrinth type seal, with convoluted fluid paths, and a very small tolerance between relatively moving surfaces, together with spaces that are enlarged and constricted so that the fluid flow is constricted and pressurized along the leakage path, thereby creating sealing properties in the configuration. This type of sealing is not conducive to efficiencies at the high rotational speeds of compressors because the tolerances must be very close and must be always maintained during the periods of rotation. Additionally, because of the ledges and other three dimensional features, these seals require an excessive amount of space in the sealing chamber, which is usually restricted by the compressor configuration. Because the rotation and relative motion of moving parts create frictional heating, even when the parts are not contacting each other, the rates of expansion of the parts may be different and cause one part to expand to the point of coming into contact with other relatively moving parts.
What is considered desirable is a combination high pressure gas and oil seal having a configuration and operation that are capable of producing a pure or almost pure process fluid, that provides for appropriate oil lubrication to the journal bearings, and while simultaneously not permitting an excessive amount of compressed gas out of the non-contacting rotary seals. Additional features that are desirable are the capability to withstand the increased temperatures created in higher rotation compressors and the like, by providing close tolerances in the seal elements, a simple, unified and axially compact design that is easily installed, that can provide excellent shaft sealing in a durable, robust configuration that is not limited by the rotational speeds of a compressor. Ideally, the sealing system does not require external control or monitoring or any support from external to the shaft seal cavity. Additional features will be described below and will become apparent to a person having ordinary skill in the art as an appreciation of the present invention is obtained.